The air con on my A4 is shot and a repair will cost more than the car is worth. German design incorporates the clutch inside the compressor so when breaks up shards of metal spread around the complete system.
My indie has just repaired a similar one and it cost £2500.

How simple to fit a clutch to the external drive shaft where it can do not catastrophic damage

<< I suspect that's based on usage, especially the lack of use of it in the winter, when presumably some will inevatbly leak as the seals/parts aren't lubricated by the system in use. >>

The obvious purpose of air-con is to cool a car in hot weather, but it can be equally (perhaps more) useful in winter to clear misting on the screen.

I find jamming the temperature down to minimum once a fortnight to a month in colder weather for about 5-10 mins, then compensating back up to max. hot for the same does the trick and keeps the air ducts free of the microscoipic nasties that can give off that musty smell and which won't do our respiratory systems any good.

An aircon repair specialist once advised me to occasionally spray a small bit of bleach into the air intake, with the aircon running, and then turn it off about 2 seconds later (ie just long enough for it to be sucked into the pipes, but hopefully before it gets into the cabin!). Then leave overnight to dry. He swore that is the best way of keeping the system clear of bacteria and mould, and just as effective as paying $$ to a garage for an aircon clean. I've never tried it (nor paid those $$), so no doubt am breathing in those nasties.

He also advised to keep it on swtiched on all year around - it'll automatically disengage if it is too cold to operate, but it's less likely to leak if it runs most of the time, and these days barely impacts fuel economy (it was different in the early days of car aircon).

It would be interesting to see how much a top-up/regas would cost, given mine (if I recall) uses R134a and not the R1234y now (wrongly) touted by manufacturers as better all-around (it may be less bad for the environment, but from most accounts is far more prone to leaks).

Had my Yaris done earlier this year at an a/c specialist, as I thought the a/c wasn't cooling very well (I've thought it not up to much since I bought the car seven years ago). It had lost around 30% R134A and cost seventy quid. It made no noticeable difference, the a/c's still carp. I could've had it done at thick twit for twenty quid less, but would you risk it?